1. Field of the invention
The invention concerns an interferometer device adapted to be integrated into a multiplex Fourier transform spectrometry imaging system (or "spectroimager").
2. Description of the prior art
A spectro-imager is moved parallel to an objectarea to be scanned, an area of ground, for example.
The principle of multiplexed Fourier transform spectro-imaging is known in itself, in particular from the document EP-0.068.922 which teaches how to use the relative movement of the terrestrial surface and an aircraft or a satellite for obtaining, thanks to light interferences a spectral analysis of a set of contiguous elementary ground areas. This avoids any relative displacement of optical components of the interferometer device. The successive contents of an array or mosaic of CCD sensors on which the interference fringes are formed are used.
This document specifically describes an interferometer device in the form of a SAVART polariscope.
It has been found advantageous, in particular for reasons connected with the received light flux, to use a MICHELSON interferometer comprising a semi-reflecting mirror (beamsplitter) at 45.degree. to the mutually orthogonal entry and exit beams. The semi-reflecting mirror splits the entry beam into two half-beams that are reflected back to the semi-reflecting mirror and between which a path difference is introduced: after the semi-reflecting mirror recombines part of each half-beam an exit beam is obtained with which an interferogram can be constructed. Because two half-beams are used the expression "two-wave interferometer" is sometimes used.
As described in U.S. Pat. No. 5,131,747 issued Jul. 21, 1992, to Cerutti-Maori and Simeoni attempts have been made to improve the spectro-imager system of the document EP-0.068.922 by using an appropriate two-wave type interferometer device. This work was specifically directed to:
providing two separate entry and exit channels;
increasing the usable energy gain, improving the contrast of the fringes and enhancing the radiometric resolution;
keeping the interferometer device and the spectro-imager as a whole as compact as possible and reducing its weight commensurately;
making the path difference independent of the aperture angle of the incident beam;
retaining the same inclinations for the mean rays of divergent waves; and
superposing the geometrical images very precisely.
A MICHELSON type (and therefore two-wave) interferometer device suitable for use in a spectroimager of the type described in the document EP-0.068.922 was already known, in the form of the device described by GIRARD in the patent U.S. Pat. No. 3.684.379.
To overcome at least some of the inherent limitations of the GIRARD device the aforementioned patent U.S. Pat. No. 5,131,747 proposes a new interferometer device for multiplex Fourier transform spectrometry imaging designed to be moved relative to an entry optical system image in a direction constituting a reference direction for the interferometer device, comprising a prismatic block of transparent material having mutually perpendicular first and second entry/exit faces meeting at an edge of the block, a semi-reflecting mirror in the block arranged at 45.degree. to the first and second entry/exit faces and passing through said edge of said block and first and second reflective surfaces respectively facing the first and second entry/exit faces through the semi-reflecting mirror; this new device was characterized in that:
there was associated with each entry/exit face a reference plane perpendicular to the entry/exit face and parallel to said reference direction,
at least one inclined dioptric separation surface was provided between one of said entry/exit faces and the associated reflective surface, producing in the associated reference plane a non-zero diopter angle relative to the entry/exit face,
the configuration and orientation of the first and second reflective surfaces were fixed relative to the block and each of these first and second reflective surfaces was a corner reflector with the concave side facing towards the associated entry/exit face with a bisecting plane parallel to said edge of the block perpendicular to said associated entry/exit face, the corner reflector surfaces having different angles of inclination in the reference planes and relative to the associated entry/exit faces.
In this way:
1--All of the energy concentrated by the entry objective lens system was usable.
2--Spurious reflections due to the optical surfaces encountered by the waves were relayed towards the entry paths and not on the exit paths; they did not affect the quality of the interference fringes.
3--The image formed by the entry objective lens system could be focused in any plane chosen by the user (who could also configure the interferometer for any position of the focal plane).
The present invention has the same objectives as the aforementioned U.S. Pat. No. 5,131,747 in the same field of application and with the same advantages but is additionally directed to improving the flexibility of use, to expanding the field of application and to further reducing the weight and volume on the equipment. Although the aforementioned U.S. Pat. No. 5,131,747 represented an undeniable advance it employed an objective lens system that was necessarily limited in terms of its numerical aperture and which additionally had to be adapted to passage through a large thickness of glass. Also, the corner reflector systems reversed the image formed by the objective lens system which necessitated rigorous straightness and alignment of the edges of the corner reflector system, which was extremely critical if the interferometer was to function correctly but very difficult to achieve.
The invention achieves the stated objective by discarding an improved MICHELSON type interferometer in favor of a totally different (McZENDER type) interferometer which those skilled in the art have until now tended to regard as having much less potential application than the MICHELSON interferometer, especially in spectro-imaging.